Our long term objective is to provide a unified theory of the evolution of interactions built on the principles of population genetics. Five projects are proposed relating to this general goal. First, I propose studying the role of informational error in the origin and evolution of the sexual cycle. This project addresses the origin and evolution of alternation of generations, which is basic to the life cycle of eukaryotes. Second, I propose studying the role of deleterious mutation in the evolution of reproductive systems. Third, I propose studying the role of variation in fitness in the evolution of communication. Recently, several hypotheses have been advanced concerning the role of fitness variation in mate choice and this project seeks to provide a common framework for the evaluation of this work. Fourth, I propose studying the effect of inbreeding on the evolution of social behavior and the effect of social behavior on the evolution of inbreeding. Fifth, I propose studying the evolution in somatic time of interactions between neurons in the brain. By combining both the evolutionary and neurobiological approaches to behavior, the proposed work studies adaptive behavior both from the point of view of ultimate or evolutionary causality and from the proximate point of view of neuronal mechanisms. The proposed work is important for the following reasons. The problems addressed, sexual recombination, animal communication, breeding system, social behavior, and category formation in the brain, are all major unsolved problems in biology. Most human diseases, pathogens and parasites are transmitted during social interactions. The proposed work studies the effects of these interactions on the biological fitnesses of individuals and how these effects have evolved.